With deregulation, telephone company customers have been able to purchase telephone equipment from a variety of sources This telephone equipment could be a single telephone or a private branch exchange (PBX) or other telephone switching system for a large corporation. The local telephone companies, however, are not required to service any subscriber-owned equipment purchased from a supplier other than the telephone company itself Consequently, when a service problem arises, the local telephone company would like to be able to determine, without the expense of a trip to the subscriber's site, whether or not the problem is caused by the telephone company's equipment and lines or by the subscriber-owned equipment.
To this end, telephone companies employ an automated subscriber loop test system, located at the central office, and remote isolation devices (RID's) connected between the telephone company's lines and the subscribers' equipment. RID's are used to momentarily isolate the subscribers'equipment from the telephone company's lines during testing by the automated loop tester to determine the source of a problem. This testing may also be done as part of a periodic, general testing of all telephone lines. An automated subscriber loop tester is described in Ashdown et al. U. S. Pat. No. 4,139,745 ("'745 patent") and Ashdown et al U.S. Pat. No. 4,113,998 ("'998 patent"), which are hereby incorporated by reference, and is commercially available from Teradyne, Inc. under the 4TEL trade designation. Other automated subscriber loop testers are commercially available from other sources. A typical testing sequence includes doing direct current (D.C.) and alternating current (A.C.) measurements, e.g., as described at col. 2, lines 25 to 45 of the '998 patent The results of the test sequence include a "fault value" (in ohms), a "type of fault" identification (e.g., a "short" between the tip and ring lines of the same telephone line, a "cross" connection of one telephone line to an adjacent line, or a "ground" connection to the tip or ring line), and an indication of whether there is a remote isolation device (RID) and, if so, the type of device. If an RID is detected, a further series of D.C. and A.C. measurements is performed in order to "segment the fault", identifying it as either a telephone company fault or a subscriber fault.
Prior remote isolation devices presently in use include voltage sensitive switch (VSS) type devices that block D.C. test signals below their thresholds and pass D.C. signals above their thresholds and pulse-activated devices that provide a high impedance open to the test system's signals after an activation pulse has been applied.